1. Field of the Invention
The present invention relates to an automobile antenna system and, more particularly, to an improved automobile antenna system for effectively detecting broadcast radio waves received by the vehicle body and then transferring detected signals to various receivers located in the vehicle.
2. Description of the Prior Art
Antenna systems are indispensable to modern automobiles which must positively receive various broadcast waves such as those for radio, television and telephone at the receivers located within the vehicle. Such antenna systems are also very important for citizen band transceivers.
One of the conventional antenna systems is known as a pole-type antenna which projects outwardly from the vehicle body of an automobile. Although such a pole antenna is superior in performance in its own way, it always remains a nuisance from the viewpoint of vehicle body design.
Furthermore, the pole antenna is disadvantageous in that it is subject to damage, tampering or theft and also in that the antenna acts to generate noises during high-speed driving. For these reasons, there has heretofore been a strong desire to eliminate the need for such pole antennas.
With the enlargement of the frequency bands for broadcast or communication waves received at automobiles in recent years, a plurality of pole antennas have been required in accordance with each frequency band. This brings about other problems; a plurality of pole antennas damages the aesthetic appearance of the automobile and the receiving performance is greatly deteriorated by electrical interference between the antennas.
Efforts have been made to eliminate the pole antenna system or to conceal the antenna from the exterior. One such proposal has been to apply a length of antenna wire to the rearwindow glass of an automobile, and this proposal has been put into practical use.
Another proposal has been to detect surface currents which are induced by broadcast waves on the vehicle body of an automobile. This seems to be the most positive and efficient way for receiving broadcast waves, but the experiments carried out to date have not provided any satisfactory results.
One of the reasons why surface currents induced on the vehicle body by broadcast waves have not been utilized well is that their induced value is not as large as expected. Although the prior art mainly uses surface currents induced on the roof panel of the vehicle body, no surface currents of a satisfactory level have been obtained.
Another reason is that surface currents contain noises of a very high level. Such noises are mainly generated by the engine ignition system and the battery charging regulator and cannot be eliminated unless the engine is stopped. Noises transmitted to the interior of the vehicle make it impossible to effect any practicably clear reception of broadcast waves.
In such a situation, some proposals have been made to overcome the above problems. One such proposal is disclosed in Japanese Patent Publication No. 22418/1978 in which electrical insulation is formed at a portion of the vehicle body on which currents are concentrated, with the currents being detected directly by a sensor between the opposite ends of the insulation. Although such structure can detect usable signals which are superior in S/N ratio, a pickup used therein requires a particular cutout in the vehicle body. This cannot be accepted in the mass-production of automobiles.
Another proposal is disclosed in Japanese Utility Model Publication No. 34826/1978 in which an antenna including a pickup coil for detecting currents in the pillar of a vehicle body is provided. This is advantageous in that the antenna can be disposed completely within the vehicle body. However, it is not practical for the pickup coil used therein to be located adjacent to the vehicle pillar in a direction perpendicular to the longitudinal axis of the pillar. Thus, it also appears that this arrangement cannot pick up any usable output from the antenna.
As has been described above, the conventional antenna systems have not been successful in efficiently detecting currents induced on the vehicle body by broadcast waves.
No effective measure has heretofore been proposed for overcoming the above-described principal problems of the conventional art in providing, in particular, a pickup structure for effectively detecting currents induced on the vehicle body by broadcast waves and a pickup arrangement capable of obtaining a usable S/N ratio. The results of various kinds of experiments show that it might in fact be basically impracticable to use an antenna system which utilizes currents flowing on the vehicle body.